Project Summary/Abstract The continuous remodeling of actin cytoskeleton, which is regulated through interactions with actin binding proteins, is vital to many cellular processes, including cell shape maintenance, migration of healthy and metastatic cells, neuronal development, synaptic plasticity etc. The main goal of this proposal is to gain insights into mechanisms of cellular actin disassembly, which are less understood than actin assembly. Actin severing by ADF/cofilins alone cannot fully account for cellular disassembly and it is assisted by other actin interacting proteins. Cofilin is implicated in cell invasion during metastasis and understanding its regulation by other proteins may broaden the range of potential therapeutic targets. In addition to general actin disassembly mechanisms, our group has long standing interests in understanding how actin remodeling is regulated in neuronal cells. Recently, we discovered that multidomain cytosolic oxidation-reduction enzyme (Mical) synergizes with cofilin to disassemble actin in axonal growth cones leading to their collapse. This is critical for neuronal pathfinding and regeneration after injury. Also poorly understood is actin remodeling in dendritic spines (DS) - actin rich dendritic protrusions, which are involved in synaptic transmission and undergo activity- dependent enlargement and shrinkage. DS are highly enriched in actin-stabilizing protein drebrin A, which affects actin assembly and disassembly rates. How is drebrin integrated with other actin regulators in DS, including formins and Arp2/3, is unknown. Our goal is to advance this knowledge. A decrease in drebrin's level in patients with neurological disorders (Alzheimer's disease, Down syndrome, epilepsy) makes it a potential therapeutic target, increasing the interest in its function. The work proposed in Aim 1 brings together many approaches - including cryo-electron microscopy, fluorescence spectroscopy and imaging, mutational work, chemical cross-linking, etc., - to gain structural understanding of the mechanism of cofilin-mediated actin disassembly and its potentiation by assisting factors (coronin and Aip1). The work proposed in Aim 2 will explore in depth a previously unknown mechanism of F-actin disassembly, the just discovered partnership of human cofilin 1 with Mical. This work combines cellular, genetic, and biochemical and structural approaches to provide mechanistic understanding of how stereo-selective oxidation by Mical of two methionine groups on actin primes it for cofilin's action, which has widespread physiological and pathological implications. Our interest in understanding neuronal actin remodeling brings the goal - in Aim 3 - of achieving mechanistic understanding of actin severing by Inverted Formin 2 (INF2), a unique formin family protein capable of both actin filaments assembly and disassembly. We identified INF2 as a potential drebrin-interacting partner, but it is also known to orchestrate mitochondrial fission and its mutations are implicated in degenerative kidney disease and peripheral neuropathy. The work proposed in Aim 4 will clarify how actin remodeling in DS occurs in a background of high concentrations of neuronal actin-binding protein drebrin A.